Most scientists think that tachyons do not exist. Einstein's theory of special relativity says nothing can accelerate faster than the speed of light, the theory is that these particles would be constantly traveling faster than the speed of light. If a tachyon did exist, it would have an imaginary number as its mass.

Many scientists believe that if one tachyon existed in the universe at any time, then the universe would be overrun by more and more tachyons. This is probably due to the fact that as they slow down, they increase energy. However, some scientists still believe that they could exist if they did not interact with normal matter.

All the particles that go below the speed of light and have mass are known as bradyons.

Curiously, the slower a tachyon moved, the more energy it would have. Therefore, the more energy a tachyon had, the slower it would go, but never to be equal to or slower than the speed of light.

It was once thought that neutrinos in experiments at the ATLAS detector in CERN might have moved faster than the speed of light, which was a problem because neutrinos do have a positive, real number mass. This lead to further investigations into more unusual theories of, for example, how particles with normal mass may move faster than light speed, and some claimed that this result would disprove general relativity. However, it was later revealed that the abnormal results were almost certainly the consequence of instrument failure, leading a wide dismissal of tachyonic behaviour in this instance, although some have remained confident that the theories used to explain the faster-than-light case would still be found accurate. Claims that these observations would disprove general relativity, which was reported probably unhelpfully widely in the press, now also seem unlikely. .